Region-specific enhancers near two mammalian homeo box genes define adjacent rostrocaudal domains in the central nervous system.

To gain insight to the mechanisms underlying region-specific gene expression in mammalian development, we investigated the regulatory DNA associated with the proximal promoter of two homeo box genes, murine Hox-1.3 and human Hox-5.1. Using lacZ gene fusions in transgenic mice, we identified regulatory elements in the 5'-flanking sequences of the Hox-1.3 and the Hox-5.1 genes that specifically direct beta-galactosidase expression to the brachial and the upper cervical regions (respectively) of the central nervous system (CNS). These two elements act at the transcriptional level, are active in either orientation, and confer region-specific expression to unrelated promoters, satisfying the criteria for enhancer elements. The two spatial domains defined by these enhancers are directly adjoining, extend along the rostrocaudal axis for the same span of 6-7 metameres, and represent specific subsets of the overall CNS regions expressing all endogenous Hox-1.3 or Hox-5.1 transcripts. The adjacent domains in the developing murine CNS that express Hox-1.3 and Hox-5.1 gene fusions are strikingly reminiscent of the adjacent stripes of expression in Drosophila embryos seen with Sex combs reduced and Deformed, the two Drosophila homeotic genes most homologous to Hox-1.3 and Hox-5.1, respectively. These findings represent the first demonstration of region-specific mammalian enhancers and raise the possibility that the mammalian CNS may be subdivided into a series of rostrocaudal domains on the basis of the activity of enhancers near homeo box genes.